U.S. Pat. No. 4 209 302 discloses the use of .alpha.- or .beta.-naphthylamine which carries, on the amino group, a propyl radical substituted by a further amino group for marking mineral oils. The marker is detected by reacting the naphthylamine with diazotized 2-chloro-4-nitroaniline. However, it has been found that this marking and detection method is not yet completely satisfactory.
Furthermore, U.S. Pat. No. 2,689,171 describes the marking of mineral oils with aniline or p-toluidine, these products being used as diazo components for the formation of azo dyes; i.e. during their detection they are diazotized and coupled with a coupling component, for example .beta.-naphthol.